1. Field of the Invention
The present invention relates to a garnet crystal for growing a bismuth-substituted rare earth iron garnet single crystal film which is used as a magneto-optic element built in optical isolators and optical switches, and more particularly to a garnet crystal substrate which has a large lattice constant and allows solid solution of a large amount of bismuth in a bismuth-substituted rare earth iron garnet single crystal film.
2. Description of the Related Art
A bismuth-substituted rare earth iron garnet single crystal (referred to as BIG) is an excellent material that exhibits good transparency and large Faraday effect in a near-infrared range. Single crystal thick films having a thickness of several hundred microns have been practically used as a Faraday rotator built in optical isolators and optical switches.
A BIG thick film having a thickness of several hundred microns is usually grown on a non-magnetic garnet substrate by a liquid phase epitaxial method. When growing such a thick BIG film, the BIG film and substrate must be very accurately aligned in lattice constant. A large difference in lattice constant results in defects in crystal structure or cracks in the substrate due to stress.
A garnet substrate for growing a BIG thick film includes an SGGG substrate made of (CaGd)3(MgZrGa).sub.5 O.sub.12 having a lattice constant of 1.2497 nm and an NGG substrate made of Nd.sub.3 Ga.sub.5 O.sub.12 having a lattice constant of 1.2509 nm. The Faraday effect and lattice constant, of a BIG film increase with increasing amount of bismuth-substitution and there have been demands towards a garnet substrate having an even larger lattice constant. An article in "Journal of Solid State Chemistry VOL. 5(1972), P. 85ff." has reported on a garnet crystal having a large lattice constant. The article reports that garnet phase was observed when a mixture of rare-earth oxide and gallium oxide reacted in solid phase.
In order to manufacture a substrate for growing a bismuth-substituted rare earth iron garnet single crystal film, a single crystal is required which has a diameter greater than one inch without cracks. However, there has been no report on such a single crystal yet.